1. Technical Field
The present invention relates to a pulse width modulation circuit which modulates an input signal, and outputs a binary, pulse width modulated signal.
2. Related Art
A pulse width modulation circuit which modulates a pulse of an input signal, and outputs a binary, pulse width modulated signal, is widely used to digitally control a voltage. With the pulse width modulation circuit, reference signals with a sawtooth wave, triangle wave, or the like responding to clock signals are generated, the reference signal and the input signal are compared with respect to magnitude, and a binary voltage signal which is at a high level when the input signal is larger than the reference signals, and at a low level when the input signal is smaller than the reference signals, is output as a pulse width modulated signal.
In this kind of pulse width modulation circuit, in order to make the pulse width modulated signal of a high resolution, it is the simplest to heighten the frequency of the clock signals which are the source of the reference signals. However, in the event of heightening the frequency of the clock signals which are the source, a calculation of a comparison between the reference signals and input signal cannot keep pace, making an internal calculation process unstable. That is, there is a limit on a balance between a high resolution and a high speed response.
Therefore, with a pulse width modulation circuit described in JP-A-4-159821, a configuration is such that, by adding or subtracting pulse width modulated signals of a plurality of pulse width modulators, a multivalued signal is generated in a voltage axis direction, and filtered by a smoothing circuit, obtaining an analog signal. Also, with a pulse width modulation circuit described in JP-A-2004-312442, a configuration is such that a plurality of sawtooth waves with differing phases are generated, the sawtooth waves and an input signal are compared and, by synthesizing outputs of results of the comparison via a resistor, a multivalued signal is generated in a voltage axis direction, and filtered by an integration circuit, obtaining an analog signal. That is, with these pulse width modulation circuits, it is intended to realize a high resolution by obtaining the multivalued signal in the voltage axis direction.
However, as a resister or switching element which connects or disconnects the outputs at each voltage stage is needed in order to output the multivalued signal in the voltage axis direction as in the pulse width modulation circuits described in JP-A-4-159821 and JP-A-2004-312442, a need occurs to install the resistor or switching element as a part in the event that power is high, as in a case of driving a digital power amplifier, and a circuit size increases by that much, increasing a cost too.